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Part 1: Existing Systems

Preventing ground faults and alerting when potential problems arise.

The demand for design and construction services for commercial solar photovoltaic (PV) systems has increased dramatically over the last several years. This increased demand has resulted in many systems being designed and built by companies with relatively little experience. In addition, increased competition has resulted in some vendors sacrificing quality and safety to reduce system design and construction cost. Even though fires are not common at commercial solar PV systems, these deficiencies could make fires more frequent. For that reason, it is even more important to make sure that fire safety is given the proper consideration during design and construction of solar PV systems.

Faults (undesired current flow paths) can occur in any electrical system, including PV systems. A fault is a malfunction in the insulation surrounding a conductor. Causes of faults include, but are not limited to, construction deficiencies, corrosion, and physical damage. Faults will have some resistance to current flow. A major fault will develop a small amount of resistance in a very short time, but faults usually have a high initial resistance compared to the normal circuit. Faults can also occur across air by generating an arc to complete the circuit. A direct current (DC) arc may have a relatively high resistance.

Since solar modules will generate power any time a sufficient light source is available, the DC section of a solar system may remain energized even after shutting down the inverters and opening the disconnects for the alternating current (AC) and DC circuits. A code compliant solar PV system does not require an automatic feature to isolate the DC wiring at the combiner level. Two or more faults in the DC circuits can cause undesired current flow paths in the system, which can cause a fire, either by overheating at the fault location(s) or by overloading circuits with excessive current flow.

The DC circuits of a large commercial solar system are equipped with a ground fault detection circuit in the inverter. This circuit typically consists of a low-current fuse that connects the DC equipment ground conductors to the grounded system conductors. A fault to ground of a DC conductor will allow current to flow through the equipment grounding conductors and return to the system through the ground fault detection circuit. Sufficient fault current flow will blow the fuse. If the fuse blows, the inverter will automatically shut down. Unfortunately, leakage currents generated in the

INTRODUCTION

“Even though fires are not common at commercial

solar PV systems, these deficiencies could make

fires more frequent”


system cause a detectable current flow in the ground fault detection circuit during normal system operation (Nelson 13-15) (Dhere, Pethe and Kaul Slide 4). This current flow can vary depending on the size of the system, module technology, the presence of moisture, or changes in humidity or temperature. Because of this variation, the level of current required to indicate a ground fault is set relatively high (typically 2 to 5 amps on large central inverter systems).

This creates a problem for large PV systems in the United States. The current version of NFPA 70 (the National Electrical Code) allows the DC circuits of a system to have one conductor grounded (NFPA 70-603). In this case, a substantial fault on the grounded conductor could easily go undetected because a fault on the grounded conductor will create a circuit parallel to the normal current flow path at the same voltage drop. The fault path will typically have significantly more resistance than the normal current flow path, and since the voltage drop is the same for both paths, the majority of the system current will still follow the normal path. The ground fault fuse will not blow, because the current that flows through the fault and the ground fault fuse will be a fraction of the total current and not enough to blow the fuse.

For example, suppose a system has a ground fault fuse rated at 5 amps. A grounded DC output circuit conductor for this system has 300 amps of current flow and a resistance of 0.2 ohms. In two parallel DC circuits, Ohm’s law tells us that the ratio of the currents in the two circuits is inversely proportional to the ratio of the resistances. Current flow through the fault must be at least 5 amps to blow the fuse. This means that the resistance at the fault must be less than 12 ohms in order for there to be sufficient current to blow the ground fault fuse.

Even if a ground fault is detected and the inverter is shut down, current can still flow in the DC circuits. Since the DC circuits are not required to be isolated except at the inverter DC bus outputs, all of the DC output and source circuits serving a single inverter are connected through the input busses and remain connected even after the inverter shuts down and the main DC disconnect is opened. This means that a combination of two or more faults to ground can allow current flow from the entire array though the faults. Fuses in the combiners may not blow in such a situation, because the current flowing through the combiners could be normal in magnitude; depending on the location of the fault, the current may actually flow backwards in parts of the circuit.

In light of this phenomenon, the purpose of this report is to present solutions to improve the reliability and safety of existing commercial solar PV systems and the buildings on which they are mounted. It will examine some conditions that cause undesired DC current flow in solar PV systems by evaluating fault incidents at several large commercial solar PV systems. Some of these faults resulted in fires. Suggested actions and modifications to improve existing systems are presented in detail. Changes to design and construction of future systems will be discussed in Part 2 of this report. The intent is for solar PV system designers, installers, operators, and owners to use this information to improve the safety of existing and future commercial solar PV systems.

“Unfortunately, leakage currents generated in the

system cause a detectable current flow in the ground fault detection

circuit during normal system operation”

“In light of this phenomenon, the purpose of this report is to present

solutions to improve the reliability and safety of

existing commercial solar PV systems and the

buildings on which they are mounted.”


Note that small PV systems, both residential and commercial, generally do not have large current leakage to ground. The ground fault detection circuits in these systems can detect ground faults of 1 amp or less. This means that small ground faults, even on the grounded conductors, can be detected easily in a small PV system. Therefore, the systems evaluated in this report are limited to large central inverter designs. However, many of the recommendations in this report are applicable to these smaller systems. While the fire risk is lower for smaller systems, it is still strongly suggested that the applicable recommendations be considered for implementation.


Examples

Several recent events at commercial solar PV systems in the US are relevant to this report. These events are summarized here. For clarity, each system is referred to by a generic name. Whenever a particular system is referenced, its generic name is underlined.

Existing Solar PV System, Commercial Building Roof Bakersfield, CA, April, 2009

A fire started at a 383 kW PV array on a large commercial building in Bakersfield, CA. While damage to the PV system was extensive, damage to the building was minimal and confined to the roof. No injuries occurred as a result of this fire. The cause of this fire was determined to be two separate faults to ground in the DC wiring. A ground fault on the grounded conductor had existed on the system for some time but remained undetected because the amount of fault current was too low to clear the ground fault fuse. On the day of the fire, a second ground fault occurred due to an improperly specified and installed conduit expansion joint. These two faults allowed the DC current generated by the entire array to pass through the grounded conductor for a single string at the location of the first fault. This current overloaded the string conductor and started the fire. A second smaller fire also started at the location of the second fault due to heating by the arc at the fault location (Brooks) (Jackson).

The post fire investigation identified several important issues:

− Even a low resistance fault on the grounded conductor may not be sensed by the ground fault detection system. This is because the normal current flow path has extremely low resistance so that very little current will flow through the fault and the ground fault detection circuit.

− Even though the ground fault fuse blew when the second fault occurred and the inverter properly shut down as a result of the ground fault, the fire was not prevented because these automatic actions did not interrupt the DC current flowing to the faults.

− The raceway for the system conductors on the roof was Electrical Metallic Tubing (EMT). Some of the compression fittings for the EMT were not properly tightened. Investigators suspected that the tubing was not inserted correctly into the fittings.

− Expansion joints in the conduit runs were insufficient to prevent thermal movement from damaging conductors and conduit.

− There was no easy way to electrically isolate the module strings from one another. Until an electrician was called to the site to remove the fuses from the combiners, a portion of the DC wiring remained energized.

BAKERSFIELD

A second ground fault due to an improperly installed

conduit expansion joint triggered the fire


Existing Solar PV System, Industrial Building Roof Carolinas, April, 2011

A fire started at a ~1 MW PV array on a large industrial building in the Carolinas. Damage to the PV system was extensive, but damage to the building itself was minimal. No injuries occurred as a result of this fire. The system owner retained PSS and engineer Bill Brooks to investigate this fire. Once again, investigators determined the cause of this fire to be two faults to ground in the DC wiring. A difference between this event and the Bakersfield event is that both faults at Site A occurred on DC output conductors that were rated for higher current flow than the string conductors. The fire appears to have started as a result of heating caused by arcing at the fault locations. As with the Bakersfield incident, there were actually two fires at Site A, one at each of the two fault locations (Brooks, Proprietary Fire Investigation Report).

The important issues discovered during this incident investigation were:

− Again, the inverter served by the faulted array properly detected a ground fault and automatically shut down as a result. However, the fire still occurred because the inverter shut down did not interrupt the DC current flow to the faults.

− Investigators discovered damage to the insulation of the DC conductors at several locations on the roof of the building. Some of the insulation damage may have occurred as a result of improper installation of the cable in the conduit, while some of the damage may have occurred because of insufficient allowances for thermal movement of the conductors and conduits causing chafing of the conductor insulation at bushings.

− The raceway system was not electrically continuous. There were many conduit couplings without bushings and there were several non-conducting junction boxes used without equipment grounding jumpers to electrically connect the conduit in the boxes. This caused the faults to travel long distances along the metallic raceway to reach a grounding point. Since there could have been additional arcing and heating where current was passing through high resistance sections of the raceway system, this situation could have led to additional fires at other locations on the roof.

− As in the Bakersfield incident, expansion joints in the conduit runs were insufficient to prevent thermal movement from damaging conductors and conduit. In addition, poor wire management may have worsened the damage to the conductors.

− Again, it was difficult to isolate the strings from one another. In this case, the fuses in the combiners were not removed under load. Instead, the fuses were removed after sunset, when the generation from the panels stopped.

SITE A

Insulation damage went undetected by the ground fault protection fuse in the

inverter


New Solar PV System, Industrial Building Roof Mid-Atlantic, May, 2011

As part of the commissioning of a new ~700 kW commercial solar PV system, the PSS commissioning team conducted insulation tests on all of the DC conductors. This testing was accomplished by lifting the conductors at the main DC busses at the inverters, and then testing each circuit using a mega ohm tester. A fault was discovered on a grounded DC output circuit conductor. The fault was insulation damage that was apparently caused by improper installation of the cable into the conduit. The system had operated while this fault existed, but the ground fault detection system did not sense this fault. The total current flowing through the ground fault detection circuit was 2.3 amps, which was insufficient to blow the fuse. This fault was repaired and the system was restored to service.

The important issues from this incident are these:

− As at Bakersfield, a fault on the grounded conductor may not be sensed by the ground fault detection system, even on a large output circuit.

− Preoperational testing of large solar systems should include insulation testing of all conductors.

− Cables should be inspected for insulation damage immediately after installation. Cables with damaged insulation should be replaced.

− Timely commissioning of the system uncovered a problem that could have led to a fire. In an existing system, preventive maintenance can detect a problem like this before it results in system damage.

SITE B

“The system had operated while this fault existed, but the ground fault detection system did not sense this

fault.”


Existing Solar PV System, Commercial Building Roof North Carolina, May, 2011

In response to the fire at Site A, PowerSecure Solar personnel were asked by the system owner to investigate Site C, which is a ~500 kW existing solar PV system located on the roof of a commercial building in North Carolina. Assisting with this investigation were Bill Brooks, PE, of Brooks Engineering, Vacaville, CA; an engineer representing an instrument manufacturer; and engineers working directly for the system owner.

The first activity was insulation testing of all of the DC conductors for the system. PSS shut down the inverter to perform the testing. This system is equipped with disconnecting combiners, so all of the DC circuits were opened at the disconnects in the combiners. Then, the DC conductors at the main DC busses in the inverter were lifted. Each of the circuits feeding the main DC busses was then tested for faults using a mega ohm tester. No faults were detected on the system. PSS then restored the system so that an additional test could be performed.

The next activity was testing of a differential current monitor (also called a residual current monitor or RCM). This device uses a current transformer (CT) to check for a difference in current between the grounded and ungrounded DC conductors. The RCM is quite sensitive, and can detect differences in current as small as 10 mA. PSS installed the CT at the DC output circuits near the DC input busses in the inverter. Then, simulated ground faults were placed on the system using jumpers with varying size resistors. The RCM was able to detect these simulated faults based on the difference in current between the grounded and ungrounded conductors.

After the successful test of the RCM, PSS permanently installed this device in the system. The monitor has 12 channels supporting 12 CTs. These CTs were installed so that they are monitoring all of the DC output circuits from the array. The CTs for this particular system are able to serve multiple sets of conductors, so a 12 channel RCM can provide ground fault detection capability for at least 24 DC output circuits. The RCM was wired to provide an e-stop signal to the inverter. An e-stop will notify the owner by sending a signal to the owner’s monitoring station.

Because of the addition of this RCM to the system, the system at Site C is now capable of detecting very small ground faults and automatically responding to a detected fault by shutting down the inverter and notifying the owner of the problem. This notification will allow the system owner to respond and repair the fault promptly, but the system owner must still take action to repair the system before a second fault occurs.

SITE C

No faults were found at Site C, and it was subsequently recommissioned with a

differential current monitor


Solutions PowerSecure Solar proposes several solutions to increase the safety of both new and existing systems. For new systems, appropriate solutions should be implemented by the system designer in the design phase. For existing systems, we propose solutions that will provide greater system safety with minor modifications.

NEW SYSTEM SOLUTIONS There are a number of steps that can be taken in new system design and construction that will provide greater protection from the types of events described above. PowerSecure Solar is already implementing many of these solutions. In part 2 of this report PSS will present these new system solutions.

EXISTING SYSTEM SOLUTIONS There are many existing systems in operation today. A significant number of these may be vulnerable to the types of events described above. PowerSecure Solar recommends that all system owners engage an experienced and qualified installer or engineer to evaluate whether the services outlined below would improve the safety and reliability of the system. PSS offers these services and is prepared to implement them.

PREVENTIVE MAINTENANCE PROGRAM By their nature, solar systems expose a large amount of electrical equipment and components to harsh outdoor conditions. This equipment is exposed to sunlight, temperature extremes, wind, precipitation, dirt, animals, and more. Because of this, even the best designed and installed systems will sometimes sustain damage.

A robust, twice annual preventive maintenance program can prevent minor system damage from becoming a fire or other significant event. These programs will identify issues that may lead to system faults, including problems with conduit, fittings, insulation, expansion joints, and other components. In addition to visual inspections, preventative maintenance should include insulation testing of all conductors that are susceptible to environmental damage.

In addition to finding and repairing damage, preventive maintenance should also include steps that will improve system performance, including cleaning and alignment checks of the modules. By using a good preventive maintenance program from a knowledgeable and skilled provider, system owners can ensure peak performance, reliability, and safety for the life of their system.

“These programs will identify issues that may lead to system

faults…”


DIFFERENTIAL CURRENT TESTER WITH CONTACTOR COMBINERS

After evaluating the background incidents, PowerSecure Solar has reached the conclusion that the best way to protect a PV system from DC circuit faults is to retrofit the system so that it takes automatic protective action if a ground fault occurs. By implementing this modification, the system will be continuously monitored for any ground fault on either the grounded or ungrounded conductors. If the system detects a ground fault, it will automatically isolate the DC circuits, stop the inverter, provide a local alarm, and notify the owner of the event.

The proposed modification also meets these factors that make it ideal for a retrofit to an existing system:

− The power output of the system is unaffected.

− The outage required for the modification is short.

− The cost of the modification is relatively small.

− All of the new components are code compliant and qualified as required.

The modification will make these changes to the system: A residual current monitor (RCM) along with current transformers (CTs) will be added to the system. The function of the monitoring package and details about installation will be identical to the modification already implemented at Site C and described in the background above. Along with installation of the monitoring package, the combiners will be replaced with contactor combiners. A contactor combiner uses an electrically held relay to close a contact on the ungrounded output of the combiner. On loss of power to the solenoid, the relay will open the contact and isolate the combiner from the rest of the DC system. The system will be arranged so if it detects a ground fault, the RCM will immediately stop the inverter, open the contacts for all of the combiners serving that inverter, provide a signal that can be used to notify the owner, and provide a local alarm.

“The system will be arranged so if it detects a ground fault, the RCM will

immediately stop the inverter, open the contacts

for all of the combiners serving that inverter,

provide a signal that can be used to notify the

owner, and provide a local alarm.”


Conclusion

Ground faults on the DC source and output circuits of a large solar PV system, while relatively rare, can cause significant damage to the system and to the building on which the system is installed. In addition to this, the existing ground fault detection systems will not detect some ground faults in large inverters currently used in the US. While poor design or installation may cause faults, the environment in which a solar system operates is harsh and can cause these faults without human error being involved.

Because of these issues, PowerSecure Solar believes it is important for all owners of large systems to implement two solutions that, together, will provide a greater level of protection from DC circuit faults. First, a robust, twice annual preventive maintenance program will monitor for problems and ensure peak system efficiency. Second, a modest modification to existing systems will provide reliable detection and automatic action to limit or prevent damage resulting from a fault. Implementing these two solutions will improve the safety and reliability of existing commercial PV systems. It is important to the industry as a whole to make PV systems as safe and reliable as possible. We urge all owners of existing systems to implement these two solutions right away.

“It is important to the industry as a whole to make PV

systems as safe and reliable as possible. We urge all

owners of existing systems to implement these two solutions

right away.”


Bibliography

Brooks, B. P. (2011). Proprietary Fire Investigation Report. Vacaville, CA: Brooks Engineering.

Brooks, B. P. (2011, February/March). “The Bakersfield Fire.” Solar Professional, pp. 62-70.

Dhere, N. G., Pethe, S. A., & Kaul, A. (2011). High Voltage Bias Testing of Specially Designed c-Si PV Modules. Golden, CO: Florida Solar Energy Center.

Jackson, P. E. (2009). [store name] Roof PV Fire of 4-5-09, [address], Bakersfield, CA. Bakersfield, CA: City of Bakersfield.

NECA. (2010). NECA/ANSI Standard 1 - Standard Practice of Good Workmanship in Electrical Construction. Bethesda, MD: National Electrical Contractors Association.

NECA. (2006). NECA/ANSI Standard 101 - Standard for Installing Steel Conduit (Rigid, IMC, EMT). Bethesda, MD: National Electrical Contractors Association.

Nelson, J. (2003). The Physics of Solar Cells. London: Imperial College Press.

NFPA. (2010). NFPA 70, National Electrical Code, 2011 Edition. Quincy, MA: National Fire Protection Association.

Steel Tube Institute. (2001). Guidelines for Installing Steel Conduit/Tubing. Glenview, IL: Steel Tube Institute of North America.

UL. (2010). UL-1741 - Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources. Camas, WA: Underwriters Laboratories, Inc.


The following illustrations are not representative of any specific solar PV system. Instead, they are diagrams intended to explain the DC wiring circuits of a typical commercial solar PV system and to show how the proposed modifications will improve the fire safety of the system. Figure 1 is a simplified diagram of a typical central inverter solar PV system. On the left are the strings, which are groups of modules wired together in series.

Two or more strings are connected together in parallel at a device called a combiner or combiner box. In this system, for simplicity, there are two strings each connected together at two combiners for a total of four strings. An actual system may have 20 or more combiners with 20 or more strings connected at each combiner. A set of strings all connected to a single combiner is commonly called a sub-array.

To the right of the combiners are DC disconnects. Disconnects serve as a way to remove power to the inverter system for maintenance or an emergency. The inverter accepts the DC electrical energy from the strings (via the combiners and disconnects) and converts it to AC electrical energy that is then used on the electric grid.

Part 1: Existing Systems

FIGURE 1: SOLAR PV SYSTEM SIMPLIFIED

DIAGRAM


Figure 2 shows the same system and some of the internal circuits in the components. The circuits from the strings are brought together at the combiners. The outputs of the two combiners are similarly brought together at the inverters. Even though the system shown has been simplified by reducing the number of strings and combiners, this diagram is sufficiently detailed to illustrate the effects of ground faults on the system.

The equipment ground for this system is shown schematically. The equipment ground is used to ground all of the non-current carrying metal components in the system. The system shown is a negative grounded system (the negative conductors are the grounded conductors). The equipment ground is connected to the grounded conductor through the ground fault detection circuit inside the inverter. This is the sole connection between the equipment ground conductors and the grounded DC circuit conductors.

FIGURE 2: SOLAR PV SYSTEM

SIMPLIFIED WIRING DIAGRAM


Examples

Several recent events at commercial solar PV systems in the US are relevant to this report. These events are summarized here. For clarity, each system is referred to by a generic name. Whenever a particular system is referenced, its generic name is underlined.

Existing Solar PV System, Commercial Building Roof Bakersfield, CA, April, 2009

Figure 3 shows a single fault on a grounded (negative) conductor. The particular conductor shown is a DC output circuit that connects one combiner with a DC disconnect. Assuming that the resistance of the fault is in the order of 20 ohms or above, the current that returns to the circuit via the ground fault fuse will be insufficient to blow the fuse. Therefore, this fault will go undetected.

BAKERSFIELD

FIGURE 3: UNMODIFIED SOLAR

PV SYSTEM WITH GAULT ON GROUNDED

OUTPUT CIRCUIT CONTROLLER


If the system from Figure 3 is allowed to operate with the fault, it is possible that a second fault could occur, this time to an ungrounded (positive) conductor. Figure 4 shows a second fault on another DC output circuit. Even a relatively high resistance fault on the ungrounded conductor will blow the ground fault fuse, and the inverter will shut down when it detects that the fuse is blown. If there is sufficient daylight the strings will still generate electricity even though the inverter is shut down. Since the energy can no longer travel through the inverter, it will now all flow through the faults, no matter what their resistance. Arrows on the above diagram show the flow of current in the circuits. Some of the current will flow through the busses in the inverter and then back out (backwards) into the DC system. This electric energy must be absorbed somewhere, and this will result in heating at both fault locations and possibly electrical damage and fires. To stop the flow of electric energy, the circuit must be broken in some way.

FIGURE 4: UNMODIFIED

SOLAR PV SYSTEM WITH TWO FAULTS

ON OUTPUT CIRCUIT

CONDUCTORS


Figure 5 shows a system that includes a residual current monitor (RCM) and contactor combiners. An existing system can easily be modified to include these devices. Note the contacts in each combiner to the right of the positive busses. The RCM will monitor the current of both the positive and negative conductors from each combiner. A ground fault will result in some of the current being diverted through the grounding system, so the RCM will detect this as a difference between the currents of the two conductors. When the RCM detects a ground fault in this way, it will send signals to shut down the inverter, notify the system owner, and open the contact in each of the combiners. When the contacts in the combiners are open, they will break the circuit and prevent any flow of energy through this fault.

FIGURE 5: MODIFIED SOLAR PV SYSTEM WITH

FAULT ON GROUNDED

OUTPUT CIRCUIT CONDUCTOR


Figure 6 shows the system from Figure 5, but with an additional fault. Recall from Figure 4 that a standard system with these faults will still generate electrical energy and send it through the faults, possibly causing a fire. In Figure 6, however, the open contacts in the combiners have broken the circuit between the strings and the faults, so no current will flow. This system will, therefore, automatically place itself in a safe condition until the faults can be repaired.

FIGURE 6: MODIFIED SOLAR PV SYSTEM WITH TWO FAULTS

ON OUTPUT CIRCUIT

CONDUCTORS


Figure 7 shows three strings feeding into a single combiner. Figures 7 and 8 demonstrate how the system will reach a safe condition following two faults on the DC source circuits serving the same combiner. Figure 7 shows a single fault on the grounded conductor for one string. Even a low resistance fault on this type of conductor will go undetected. Only a fraction of the circuit current will flow through the ground flow path, while the current flow for a single string (typically in the range of 6 to 8 amps) will not be enough to trip the ground fault fuse. However, the system shown in Figure 7 is equipped with an RCM, so this fault will be detected, and the RCM will send signals to shut down the inverter, notify the system owner, and open the contact in each of the combiners. Since the contact has isolated the positive conductors from the rest of the circuit, the system will stop generating electrical energy when the contacts open.

FIGURE 7: SUB-ARRAY FROM

MODIFIED SYSTEM WITH SINGLE

FAULT


Figure 8 shows the same system from Figure 7 with the first fault and the open contactor in the combiner box, but now there is also a second fault on one of the ungrounded conductors on another string. Since this second fault is on a string that feeds the same combiner as the first faulted string, it has created a circuit through the ground faults. The open contact in the combiner cannot break this circuit, but it does prevent energy from the other combiners in the system from flowing to these faults.

Also, even though the contactor is not protecting this circuit, there is twice the normal amount of current flowing backwards through the fuse on the faulted ungrounded conductor. If there is enough sunlight, the resulting current flow will blow the fuse. Even if there is insufficient current flow to blow the fuse, the RCM will notify the owner of the ground fault, giving the system owner some time to respond to the events before the system or any structures are damaged.

FIGURE 8: SUB-ARRAY FROM

MODIFIED SYSTEM WITH DOUBLE

FAULT


Figure 9 shows a before and after schematic diagram of a large central station inverter. The “existing” electrical overview shows a normal negative grounded inverter. The “new” electrical overview shows the same inverter with the residual current monitor installed. A CT that measures the current flows to and from a combiner is also shown. One CT is required to monitor every one or two combiners. Also shown are the signals that the RCM will send when a ground fault is detected. The E-stop signal to the inverter will open both the AC and DC circuit breakers and the main contactor in the inverter. The other signal will remove power to the solenoids controlling the combiner contactors, and all of those contactors will open. This makes the system safe when it detects a ground fault.

FIGURE 9: SAFETY

UPGRADES TO INVERTER


GLOSSARY This glossary provides simple definitions for some of the terms used in this attachment and in the main article:

ARRAY A group of strings consisting of modules. Array usually refers to all of the modules for a system. All of the modules that connect to a single combiner are sometimes referred to as a sub-array.

CIRCUIT Conductors arranged to allow current flow. A circuit must form one or more continuous loops in order to produce current flow. Certain electrical devices, like a module or an inverter, can form part of a circuit.

COMBINER (OR COMBINER

BOX)

A junction box where wires from individual strings are combined into larger wires to run to the inverter. A combiner typically contains fuses for the ungrounded conductor for each string.

CONDUCTOR A component that allows current to pass through it. Electrical cables are conductors.

CONTACT An electrical device that can be opened or closed, breaking or completing a circuit, and thus preventing or allowing current flow.

CURRENT The flow of electrical energy in a circuit. Current is measured in Amps. Current is normally measured at a point in a circuit.

DC DISCONNECT An electrical switch used to interrupt the circuit at the ungrounded conductor.

FAULT A malfunction in the insulator protecting a conductor that results in undesired current flow.

FUSE An electrical device that will melt or “blow” at a predetermined current level, thus opening a circuit. Used for circuit protection in the event of undesired high current.

GROUND Literally, the earth. Electrical circuits are grounded by being attached to the earth via buried metal objects like rods, rebar, plumbing pipes, or building framing. The metal components of an electrical system are always grounded, so a ground fault may occur between a conductor and another component of the electrical system (a conduit or junction box, for example).

GROUNDED CONDUCTOR

A conductor that is at the same potential (voltage) as ground. A properly grounded conductor will always be at zero (0) volts of potential.

INSULATOR A material that resists current flow, like rubber, plastic, or glass. Insulation typically surrounds and protects certain conductors like wires or electrical cables.


INVERTER The device that accepts the DC power from the modules and converts it to AC power that electric grid uses.

MODULE A single solar panel.

POTENTIAL The potential for producing current. Potential is measured in Volts.

PV OUTPUT CIRCUIT

The conductors that connect the combiner to the inverter.

PV SOURCE CIRCUIT

The conductors that connect the modules together into a string and connect the string to the combiner.

STRING A group of modules wired together in series (one module is wired to the second module; the second is wired to the third, and so on).

UNGROUNDED CONDUCTOR

A conductor that is not directly attached to ground. An ungrounded conductor usually has a different voltage (either positive or negative) compared to ground.

VOLTAGE DROP The difference in potential between two points in a circuit.

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